When deploying wireless communications systems such as, for example, cellular systems, in indoor environments in general, so called “leaky cables” are sometimes used, also sometimes referred to as leaky feeders or radiating cables.
A leaky cable is a cable which is capable of conducting electromagnetic radio frequency energy, and which has been provided with apertures in order to make the cable radiate, i.e. to allow some of the energy to “leak” from the cable, thus enabling the cable act as an antenna. Such an antenna, i.e. a leaky cable, will due to reciprocity be able to act equally well as a receiving as a transmitting antenna. Due to its nature of a cable, a “leaky cable antenna” will, as compared to a traditional antenna, act more like a line source than a point source, obtaining a more uniform coverage level compared to a point source antenna from which the radiated power falls off rapidly with distance, thus making it easier to obtain coverage in tunnels, along railways or where a high degree of “shadowing” occurs when using a point source antenna. An example of the latter is an indoor scenario, e.g. an office landscape.
A leaky feeder is typically designed as a coaxial cable or a waveguide where the outer conductor is perforated in order to create holes or slots through which some of the energy in the cable can escape and radiate into free space. Various designs exist for the slot geometry and separations. The slots can be uniformly distributed along the length of the cable or clustered in groups, thereby providing different radiating properties. Variations of the slot structure, shape, and density along the cable allow a cable designer to shape how much the cable is radiating from different sections and also in what directions. The latter property is realized through selecting on which side of the cable the slots are placed, as each slot will have directional radiation properties that essentially form a lobe or beam away from the cable.
It has been found through measurements and numerical simulations that a leaky feeder will have its radial radiation maximum in the direction that the slots are facing. More importantly, depending on the frequency and slot separation, the maximum radiation will be in a cone at a certain polar angle from the longitudinal axis. When the radiation has its maximum along the cable it is said to operate in the coupling mode, while when the maximum is more perpendicular to the cable it is said to operate in the radiating mode. FIG. 1a illustrates the cone angle of radiation from a leaky cable in coupling mode and FIG. 1b illustrates the cone angle of radiation from a leaky cable in radiating mode.
While the leaky cable is well suited to achieve good coverage in the vicinity of the cable such as in indoor or underground deployments, it can be difficult to use it to provide coverage over wider areas due to the very high directivity that the cable has in the far field. A conical beam may also not be well suited to the coverage area. Prior art antennas which are more point source-like are preferably used in such scenarios, even though these antennas have limited degrees of freedom in shaping the radiation pattern due to the compact size. Regular antennas also rely on good impedance and radiation resistance matching in order to be effective radiators. Thereby they become sensitive to detuning due to e.g. objects or persons in the near field or in contact with the antenna.